One idea could be that the female Saiyan is from a different part of the Saiyan planet and has unique powers. She and Goku meet during a universal tournament. Goku is initially intrigued by her strength and different fighting style. As they train together, they start to develop feelings for each other. Their relationship could be tested when a powerful enemy threatens the universe and they have to combine their powers and trust in each other to defeat it.
A possible plot could be that the female Saiyan is a time - traveler. She comes from a future where Goku has disappeared. She travels back in time to meet him and try to prevent whatever it was that made him vanish. Along the way, she discovers that Goku is not what she expected. He's much more carefree and kind - hearted. As they spend time together, they form a bond and fall in love, and she realizes that she has to protect this version of Goku no matter what.
A great plot could involve a Female Saiyan who has lost her memory. Goku finds her and tries to help her regain her memories. As they journey together, she starts to remember bits and pieces of her past, which are related to a great danger that she was trying to prevent. Goku and the Female Saiyan then have to work together to stop this danger before it's too late. Along the way, they form a strong bond not just as comrades but also as friends who understand each other's Saiyan nature.
One idea could be that the female Saiyan is from a different part of the universe and she and Gohan meet during a galactic tournament. They start as rivals but gradually develop feelings as they see each other's strength and honor.
One idea could be that Goku and Goku Black are forced to team up against an even greater threat from another universe. They start to understand each other's fighting styles during the battle and gradually develop a grudging respect for one another.
A possible plot could involve an evil force that targets Goku and Female Vegeta specifically because of their combined power. They are constantly on the run and have to rely on each other. In the process, they discover that their ki energies can be combined in a unique way. This discovery not only helps them fight the enemy but also deepens their bond as they realize how well they complement each other.
Maybe the goddess has a prophecy about Goku being the key to saving her realm from an ancient evil. So she forms a bond with Goku, and as they get to know each other better, Goku starts to develop feelings for her. Their relationship then becomes a driving force for him to fulfill the prophecy and defeat the evil, all while exploring the unique aspects of their two different worlds.
In a fanfiction, there could be a scenario where an ancient prophecy foretells of a combined power of Trunks and Goku being the only thing that can stop an impending doom. They start training together to master a special fusion technique that no one has ever attempted before. Along the way, they face challenges not only in their training but also in dealing with their own egos and differences in fighting styles.
A really interesting plot could be set in an alternate universe where Goku and Launch are arranged to be married for some political reason in the Saiyan or Earthling world. They start off not really liking each other but as they face challenges together, they fall in love. For example, they have to deal with a group that is against their union and in the process, they learn about each other's strengths and weaknesses.
Perhaps in a different universe, Turles and Goku are brothers. They grow up together, training side by side, but then a prophecy divides them as they are told one must defeat the other for the greater good. This creates a complex relationship full of love, rivalry, and moral dilemmas.
One idea could be that the female Saiyan is from a different part of the universe. She's heard tales of Goku's power and comes to Earth to challenge him. But when she meets him, she realizes he's not just about power but also has a kind heart. So instead of fighting, they start training together.
In some versions, the female Goku could be part of a prophecy. The two Gokus meet, and at first, there is some confusion and misunderstanding. But as they get to know each other, they discover that they are key to fulfilling the prophecy. They combine their powers and defeat an ancient evil that was threatening both of their worlds.